1. Field of the Invention
The present invention relates to a thermoplastic resin composition having a good and durable antistatic property.
2. Description of the Related Art
In general, plastics have a large electric resistance value and are readily charged by friction, peeling or the like, and thus attract dust and dirt which spoil the appearance thereof, and accordingly, various problems arise in the fields of molded articles, films, sheets, fibers and the like.
As the means for imparting an antistatic property to plastics, the following two methods are generally adopted.
(1) Internal kneading of an antistatic agent.
(2) Surface coating with an antistatic agent.
Furthermore, there can be mentioned a method of surface coating with a silicon compound, and a method of a modification of the surface structure of plastics, i.e., a plasma treatment method, but these methods are expensive.
A low-molecular-weight surface active agent is generally used in the method of internal kneading of an antistatic agent, but according to this method, it is difficult to impart a durable antistatic property, and if the antistatic agent present on the surface is removed by water washing, rubbing or the like, the antistatic property is lost. A method using an antistatic rubber is proposed in Japanese Unexamined Patent Publication No. 55-36237, but, in this method, the antistatic rubber must be incorporated in a large amount, resulting in a reduction of the inherent properties of the plastics, such as a high heat resistance, and the surface resistance value is at best about 10.sup.11 .OMEGA..
The method of surface coating an antistatic agent gives a good antistatic property, such as a surface resistance value of about 10.sup.9 .OMEGA., but this antistatic property is easily lost by water washing, rubbing or the like. Furthermore, unlike the internal kneading method, this method requires a surface coating step, resulting in an increase of the manufacturing cost.
The inventors previously proposed an antistatic thermoplastic resin composition formed by incorporating a cationic copolymer having a quaternary ammonium salt into a thermoplastic resin (Japanese Patent Application No. 61-196858), but when molded at a high temperature of higher than 220.degree. C., this thermoplastic resin composition is discolored and becomes a light yellow while the composition remains in a molding machine.
An antistatic material having a high reliability is required for a container or package for electronic parts, but as pointed out hereinbefore, an antistatic agent capable of providing a good and durable antistatic property when used in a small amount without a special step, which does not lower inherent properties of the base polymer, is not known, and the development of such an antistatic agent is urgently required.